Re: [PATCH RFC 1/9] memremap: add ZONE_DEVICE support for compound pages

From: Joao Martins <joao.m.martins@oracle.com>
To: Matthew Wilcox <willy@infradead.org>, John Hubbard <jhubbard@nvidia.com>
Cc: linux-mm@kvack.org, linux-nvdimm@lists.01.org,
	Jason Gunthorpe <jgg@ziepe.ca>,
	Muchun Song <songmuchun@bytedance.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH RFC 1/9] memremap: add ZONE_DEVICE support for compound pages
Date: Wed, 9 Dec 2020 13:12:57 +0000	[thread overview]
Message-ID: <8df7f242-9044-a3a2-0788-69328d5909a1@oracle.com> (raw)
In-Reply-To: <20201209063350.GO7338@casper.infradead.org>

On 12/9/20 6:33 AM, Matthew Wilcox wrote:
> On Tue, Dec 08, 2020 at 09:59:19PM -0800, John Hubbard wrote:
>> On 12/8/20 9:28 AM, Joao Martins wrote:
>>> Add a new flag for struct dev_pagemap which designates that a a pagemap
>>
>> a a
>>
Ugh. Yeah will fix.

>>> is described as a set of compound pages or in other words, that how
>>> pages are grouped together in the page tables are reflected in how we
>>> describe struct pages. This means that rather than initializing
>>> individual struct pages, we also initialize these struct pages, as
>>
>> Let's not say "rather than x, we also do y", because it's self-contradictory.
>> I think you want to just leave out the "also", like this:
>>
>> "This means that rather than initializing> individual struct pages, we
>> initialize these struct pages ..."
>>
>> Is that right?
> 
Nop, my previous text was broken.

> I'd phrase it as:
> 
> Add a new flag for struct dev_pagemap which specifies that a pagemap is
> composed of a set of compound pages instead of individual pages.  When
> these pages are initialised, most are initialised as tail pages
> instead of order-0 pages.
> 
Thanks, I will use this instead.

>>> For certain ZONE_DEVICE users, like device-dax, which have a fixed page
>>> size, this creates an opportunity to optimize GUP and GUP-fast walkers,
>>> thus playing the same tricks as hugetlb pages.
> 
> Rather than "playing the same tricks", how about "are treated the same
> way as THP or hugetlb pages"?
> 
>>> +	if (pgmap->flags & PGMAP_COMPOUND)
>>> +		percpu_ref_get_many(pgmap->ref, (pfn_end(pgmap, range_id)
>>> +			- pfn_first(pgmap, range_id)) / PHYS_PFN(pgmap->align));
>>
>> Is there some reason that we cannot use range_len(), instead of pfn_end() minus
>> pfn_first()? (Yes, this more about the pre-existing code than about your change.)
>>
Indeed one could use range_len() / pgmap->align and it would work. But (...)

>> And if not, then why are the nearby range_len() uses OK? I realize that range_len()
>> is simpler and skips a case, but it's not clear that it's required here. But I'm
>> new to this area so be warned. :)
>>
My use of pfns to calculate the nr of pages was to remain consistent with the rest of the
code in the function taking references in the pgmap->ref. The usages one sees ofrange_len
are are when the hotplug takes place which work at addresses and not PFNs.

>> Also, dividing by PHYS_PFN() feels quite misleading: that function does what you
>> happen to want, but is not named accordingly. Can you use or create something
>> more accurately named? Like "number of pages in this large page"?
> 
> We have compound_nr(), but that takes a struct page as an argument.
> We also have HPAGE_NR_PAGES.  I'm not quite clear what you want.
> 
If possible I would rather keep the pfns as with the rest of the code. Another alternative
is like a range_nr_pages helper but I am not sure it's worth the trouble for one caller.

	Joao
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